A number of techniques for creating a variety of patterns on the surface and inside of transparent substrates using pulsed laser radiation are well known.
One publication disclosing such techniques is the Russian invention #321422 to Agadjanov et al., published on Nov. 16, 1970 (#140454529-33). The invention concerns a method of manufacturing decorative products inside a transparent material by changing the material structure by laser radiation. As disclosed, by moving a material relative to a focused laser beam, it is possible to create a drawing inside the material.
U.S. Pat. No. 4,092,518 to Merard discloses a method for decorating transparent plastic articles. This technique is carried out by directing a pulsed laser beam into the body of an article by successively focusing the laser beam in different regions within the body of the article. The pulse energy and duration is selected based upon the desired extent of the resulting decorative pattern. The effect of the laser is a number of three dimensional "macro-destruction" (fissures in the material of the article) appearing as fanned-out cracks. The pattern of the cracks produced in the article is controlled by changing the depth of the laser beam focus along the length of the article. Preferably, the article is in the form of a cylinder, and the cracks are shaped predominantly as saucer-like formations of different size arranged randomly around the focal point of the optical system guiding a laser beam. The device used to carry out this technique is preferably a multi-mode solid-state, free-running pulse laser used in conjunction with a convergent lens having a focal length from 100 to 200 mm.
U.S. Pat. No. 4,843,207 to Urbanek et al., discloses a method of creating controlled decorations on the surface of a hollow symmetrical transparent article. This technique is preferably carried out on glass. The glass is preconditioned with a coating on the outer surface of the glass being approximately 1.2 mm thick and made of a material having at least 75% absorption of laser radiation. The technique is also carried out using a laser having a wave of length of 0.5 to 2 microns acting upon the external coating through the wall of the cylindrical glass article. The laser beam moves so that it is focused on the surface of the cylinder, and moves about the axis of symmetry of the cylinder to irradiate the aforementioned surface coating. As a result, the irradiated portions of the surface coating go through a phase change and a pattern is formed.
U.S. Pat. No. 5,206,496 to Clement et al. discloses a method and apparatus for providing in a transparent material, such as glass or plastic, a mark which is visible to the naked eye or which may be "seen" by optical instruments operating at an appropriate wavelength. The Clement et al. Patent describes a method and apparatus for producing a subsurface marking which is produced in a body such as bottle, by directing into the body a high energy density beam and bringing the beam to focus at a location spaced from the surface, so as to cause localized ionization of the material. In the preferred embodiment the apparatus includes a laser as the high energy density beam source. The laser may be a Nd-YAG laser that emits a pulsed beam of laser radiation with a wavelength of 1064 nm. The pulsed beam is incident upon a first mirror that directs the beam through a beam expander and a beam combiner to a second mirror. A second source of laser radiation in the form of a low power He-Ne laser emits a secondary beam of visible laser radiation with a wavelength of 638 .mu.m. The secondary beam impinges upon the beam combiner where it is reflected toward the second reflecting surface coincident with the pulsed beam of laser radiation from the Nd-YAG laser. The combined coincident beams are reflected at the reflecting surface via reflecting two other surfaces to a pair of movable mirrors for controlling movement of the beam. The beam then passes through a lens assembly into the body to be marked.
Soviet patent publication 1838163 to P. V. Agrynsky, et. al discloses a process for forming of image in solid media by the way of processing of optically transparent solid material by the beam of radiation with changeable energy for creation of the image.
Russian patent publication RU 20083388 C1 to Oshemkov discloses a process for laser forming of images in solid media by the way of focusing of laser radiation inside the volume of sample which differs by the following: with the aim to save the surface and the volume of the sample before the definite point and creation of three dimensional images, the sample is illuminated with the power density higher than the threshold of volume breakdown and moving the sample relatively to the laser beam in three orthogonal directions.
WIPO Patent Document No. 96/30219 to Lebedev et al. discloses a technology for creating two- or three-dimensional images inside a polymer material using penetrating electromagnetic radiation. The technology can be used for marking and for producing decorative articles and souvenirs. Specifically, laser radiation is used as the penetrating radiation, and carbonizing polymers are used as the polymer material. By these means, it is possible to produce both black and half-tone images in the articles.
U.S. Pat. No. 5,575,936 to Goldfarb discloses a process and apparatus where a focused laser beam causes local destruction within a solid article, without effecting the surface thereof. The apparatus for etching an image within a solid article includes a laser focused to a focal point within the article. The position of the article with respect to the focal point is varied. Control means, coupled to the laser, and positioning means are provided for firing the laser so that a local disruption occurs within the article to form the image within the article.
U.S. Pat. No. 5,637,244 to Erokhin discloses a technique which depends on a particular optical system including a diffraction limited Q-switched laser (preferably a solid-state single-mode TEM.sub.00) aimed into a defocusing lens having a variable focal length to control the light impinging on a subsequent focusing lens that refocuses the laser beam onto the transparent article being etched. The laser power level, operation of the defocusing lens, and the movement of the transparent article being etched are all controlled by a computer. The computer operates to reproduce a pre- programmed three-dimensional image inside the transparent article being etched. In the computer memory, the image is presented as arrays of picture elements on various parallel planes. The optical system is controlled to reproduce the stored arrays of picture elements inside the transparent material. A method for forming a predetermined half-tone image is discloses. Accordance to the method microdestructions of a first size are created to form a first portion of the image and microdestruction of a second size different from the first size are created to form a second portion of the image. Different sizes of microdestructions are created by changing the laser beam focusing sharpness and the radiation power thereof before each shot.
U.S. Pat. No. 5,575,936 to A. Vasiliev and B. Goldfarb discloses a method for laser-assisted image formation in transparent specimens which consists in establishing a laser beam having different angular divergence values in two mutually square planes. An angle between the plane with a maximum laser beam angular divergence and the surface of the image portion being formed is changed to suit the required contrast of an image.
EPO Patent Document 0743128 to Balickas et al. disclose a method of marking products made of transparent materials which involves concentration of a laser beam in the material, which does not absorb the beam, at a predetermined location, destruction of the material by laser pulses and formation of the marking symbol by displacement of the laser beam. Destruction of the material at that location takes place in two stages. In the first stage, the resistance of the material to laser radiation is altered, while, in the second stage, destruction of the material takes place at that location.
U.S. patent application Ser. No. 08/643,918, U.S. Pat. No. 6,087,617 to Troitski et al. discloses a computer graphic system for producing an image inside optically transparent material. An image reproducible inside optically transparent material by the system is defined by potential etch points, in which the breakdowns required to create the image in the selected optically transparent material are possible. The potential etch points are generated based on the characteristics of the selected optically transparent material. If the number of the potential etch points exceeds a predetermined number, the system carries out an optimization routine that allows the number of the generated etch points to be reduced based on their size. To prevent the distortion of the reproduced image due to the refraction of the optically transparent material, the coordinates of the generated etch points are adjusted to correct their positions along a selected laser beam direction.
Images comprising laser- generated etch points produced by all aforementioned systems and methods generally suffer from having a low quality. The quality of laser-induced damage images is determined by following principal parameters:
1) spatial resolution; PA0 2) adequate reproduction of grade shades PA0 3) dispersion character of light reflected from a laser- induced damage; PA0 4) artistic effect.
These parameters are determined by form and sizes of each laser induced-damage. Commonly, those etch points that are produced in glass by aforementioned methods, have random star forms in plane perpendicular to the direction of the laser beam. Such form of laser induced-damage is not desirable because it does not give a chance to produce high quality images.
The first, the random etch points have random effective sizes. The distance between adjacent damages should be larger if sizes of etch points is larger (only in this case it is possible to avoid an internal split between several separate etch points). Therefore it is necessary to produce etch points with the large distance between them to have a low probability of an internal split. This demand gives rise to low spatial resolution.
The second, the etch points with random shape have random area of modified refraction index and this gives fluctuation of light reflected from them, consequently, adequate reproduction of grade shades is impossible.
The third, the etch points with random shape have random light dispersion character, consequently, contrast of any image fluctuates.
The fourth, both brightness fluctuation of one etch point for different direction and brightness fluctuation between separate damages are visible by naked eyes and it creates undesirable (not artistic) effect.
Moreover, generally different grade of shades are reproduced by variation of laser damage sizes. Different sizes of microdestructions are created by changing of the focusing sharpness (U.S. Pat. No. 5,637,244 to Erokhin, U.S. Pat. No. 5,575,936 to A. Vasiliev and B. Goldfarb), the radiation power (U.S. Pat. No. 5,637,244 to Erokhin, Soviet patent publication 1838163 to Agrynsky, et. al) or repeat of laser pulses in the same point (Soviet patent publication 1838163 to P. V. Agrynsky, et. al). In all cases etch points are changed in size and consequently reproduction of grade shades influences on the spatial resolution of an image (variation of damage size necessitates variation of distance between adjacent etch points). Just therefore U.S. Pat. No. 5,637,244 to Erokhin disclose a method only "for forming a predetermined half-tone (not grade shade) image inside a transparent material". Moreover each image has its minimal resolution which is needed for reproduction of its small details, and therefore the increase of size has its limitation for each image. At last, it is not possible to reach high artistic effect when the same image is produced by etch points with random sizes.